XDA member itsmikeramsay has released unofficial CM10.1 ROMs for Galaxy S3 owners on T-Mobile and AT&T handsets. Whilst there are still some bugs to be worked out, it's a great start for users looking to run Android 4.2 on their S3.
Android 4.2-based CM 10.1 for Galaxy S3 and Xperia T teased, to reach nightly status soon
We’ve known for a while that CyanogenMod 10.1, based on Android 4.2 Jelly Bean, is a work in progress. However, seeing as the first ports for Nexus devices have only been made available this past week, we would have never suspected other builds are very close to reaching nightly status.